What is Timecode?
This explanation was written by Alistair Jackson of EditHouse for use in various training courses.
When editing or replaying material from multiple devices, it is usually necessary to maintain a direct time relationship between the devices. This process is known as synchronising the devices. Synchronisation is necessary in cinemas where the vision and audio information are often held on different reels or on a film reel and an audio CD. It is also very important when editing video, because this process requires the identification of specific video frames (ie. to 1/25th or 1/30th of a second accuracy).
Time and Control Code, more commonly called Timecode, is a video synchronising standard which is based on a 24-hour clock readout. Each frame of vision is given a unique Timecode reading. For example, a timecode readout of 01:12:38:06, would be read as one hour, twelve minutes, thirty-eight seconds and six frames. This readout could either be "time of day timecode", meaning the material was recorded at twelve past one in the morning, or "relative timecode", generally meaning that the recording occurred one hour and twelve minutes into the tape. Sometimes the hour counter is used to signify the tape number. Thus, 01:12:00:00 would be 12 minutes into tape number 1 and 03:12:00:00 would be 12 minutes into tape 3.
The maximum number that can occur in the Frames readout will depend on the video standard that is being used. The PAL format used in Australia and much of Europe, creates a sense of motion by animating 25 still pictures per second. Thus the frames counter can read from 0 to 24. The NTSC format used in North America and Japan consists of 30 frames per second. Thus the frames counter may read any number between 0 and 29. Timecode is also used in the film industry, where the cinema picture is made up of 24 pictures per second. Thus the frames timecode counter could read anywhere from 0 to 23.
To understand why timecode is necessary, think about what happens if you eject a VHS cassette half way through. When you put it back in the VCR, the counter will have reset to zero. That is because the VHS standard doesn't allow for the recording of timecode. All the VCR can do is count the number of video frames (or more likely count control pulses), and update its internal timing accordingly. If the tape is ejected, the counter goes back to zero. If the tape is worn or the player has dirty heads, then the counter may well miss some pulses and the time will be wrong.
Thus, if someone asks you for the location of a particular scene on a VHS cassette, it's necessary to confirm that you both started the tape from the beginning. In they say that the want scene that occurs 35 minutes and 20 seconds into the tape, then you need to rewind the tape, reset the counter (often by ejecting and re-inserting the tape), the fast-forward to 35:20. If both VCR's are in good enough condition to keep an accurate tick count when in fast-forward, then you should end up at roughly the same location on the tape. However, if you're looking for an exact frame (eg. your client is referring to a particular expression on an actors face, that is only on the screen for a fraction of a second), then there is no way to identify it on a VHS system.
The process of video editing often involves noting the timecode, either on paper or with an automated computer system, of specific frames at which video edits are to occur. The resulting list of edit points is called an "Edit Decision List" (EDL) and is used to assist in the editing process. It is particularly common to use cheaper video equipment to perform "off-line editing", which results in the generation of an EDL, and then to move to a more expensive "on-line" suite after all the editing decisions have been made.
The above example of a simple EDL shows a PAL video edit that involves assembling 4 video clips and accompanying audio. Edit 1 takes 3.5 seconds of video and audio material from a videotape designated as Reel 015 and identifies it as frames 01:00:00:00 to 01:00:03:12 for our project. Edit 2 takes just under 4 seconds of video from Reel 011. Edit 3 takes just the video frames 01:00:02:16 to 01:00:13:09 from Reel 015, and runs this almost 11 seconds into our project, starting from 01:00:07:10. Edit 4 takes audio only from Reel 007 and matches it to the 11 seconds of edit 3. Edit 5 is video only from Reel 015 and edit 6 is matching audio from Audio Track One of Reel 011. Those 6 edit decisions comprise the EDL for a 30 second video edit.
|Edit 1||Edit 2||Edit 3 & 4||Edit 5 & 6|
|Video:||Reel 015||Reel 011||Reel 015||Reel 015|
|Audio A1:||Reel 015||Reel 011||Reel 007||Reel 011|
|Audio A2:||Reel 015||Reel 011||Reel 007||none|
This example could be the EDL for a "Paper Edit" made by someone using a pen and paper and a VHS cassette with BITC (see below), or it could be a simplified excerpt from a computer Non-Linear Editing (NLE) system (such as Final Cut Pro). A full EDL from an NLE would contain all this information as well as additional information about transitions, audio levels, etc. For example, it would need to identify whether the single audio channel specified in edit 6 was to remain as channel A1, or be panned between the left and right channels.
Longitudinal Timecode (LTC) is where a dedicated linear track, sometimes a spare audio track, is used to hold one timecode address for every video frame. LTC cannot be replayed when the tape is stopped or moving slowly.
An alternative method for storing timecode is one in which the data is stored in the video field itself. This is called Vertical Interval Timecode (VITC) because the data is recorded using a video line within the vertical blanking period. VITC can be recovered when the tape is stopped or in slow motion, however it cannot be reliably recovered at high shuttle speeds because the rotary heads do not play back the entire track. When in fast shuttle, most VTRs will simulate timecode by counting control track pulses.
Another common form of timecode is Burnt-In Timecode (BITC). This is where the timecode reading is shown visual over the video picture (usually as a black rectangle in the lower centre portion of the screen, with the timecode numbers shown in white). It is important to distinguish the difference between BITC that is actually recorded on a tape, and BITC that is simply being generated by a Videotape Recorder and shown on the monitoring screen. Thus, most profession tape decks will provide the option to display the LTC or VITC they are reading from the tape as BITC on their monitoring output. Even though you can see the timecode burnt into the image, it is not actually recorded on the tape in that fashion. If you now go and record that video signal (the one with BITC that's showing on the monitor), you now have a tape with the BITC recorded on it.
Once the timecode is "burnt-in" to the video image and recorded on a tape, it is impossible to recover the visual image that the BITC is now covering. Thus, the videotape is no longer usable as a production source. BITC is most often recorded for VHS dubs, so that someone viewing the VHS copy can identify locations on a tape that they want to later correlate with the master recording.